1. Field of the Invention
The present invention relates in general to an optical disc device. More specifically, the present invention relates to an optical disc device capable of recording data on an erasable optical disc.
2. Description of Related Art
There are several types of recordable optical discs, such as a write-once optical disc and an erasable optical disc. The write-once optical disc such as a compact disc-recordable (CD-R) and the erasable optical disc such as a compact disc-rewritable (CD-RW) or a digital versatile disc-rewritable (DVD-RW) have pregrooves for guiding on the recording optical discs. The pregrooves are wobbled slightly along the radial direction of the optical disc with a central frequency of 22.05 KHz. The address data in recording, termed as absolute time in pregroove (ATIP), are recorded in a multiplex mode by FSK modulation with a maximum deviation frequency of ±1 KHz.
In regards to the tracking and focusing servo circuits of the optical disc device for recording and reproducing data on/from the optical disc, light beams are illuminated on the optical disc and the light beams reflected by the optical disc are detected by a number of photo detectors. According to a preset algorithm, the detected signals are used for generating the servo signals, which are used for driving an actuator to perform the tracking and focusing servo controls.
In regards to the optical disc device for recording and reproducing data on/from the erasable optical disc, such as the CD-RW, a 4T recording signal (EFM signal) is recorded as shown in FIG. 8A Referring to FIG. 8B, a recording signal value of “0” of the power of the light beam corresponds to the erase power Pe, and a recording signal value of “1” corresponds to the write power Pw and the read power Pb, wherein Pw and Pb alternate. In addition, the base time interval T is a period in a frequency of 4.32 MHz and of about 230 nsec.
Therefore, when data is reproduced, the incident light beams, which have the read power, are reflected by the optical disc and the reflected light beams are detected by the photo detectors and according to the detected signal, a tracking error signal is generated. When the data is recorded, the incident light beams, which have the write power and the erase power, are reflected by the optical disc and the reflected light beams are detected by the photo detectors and the sampling/hold circuits sample and hold the detected signal when the incident light beams have the erase and according to the sampling/hold signal, a tracking error signal is generated.
FIGS. 9 and 10 are exemplary circuit diagrams of a conventional servo signal generator. Referring to FIG. 9, spots of the light beams are illuminated on the optical disc and the light beams reflected by the optical disc are detected by four divided photo detectors 10A, 10B, 10C and 10D. The respective detected signals by the photo detectors 10A, 10B, 10C and 10D are transmitted to the corresponding sampling/hold circuits SA, SB, SC and SD. Based on the sampling/hold signal from the terminal 12, for example, a high-level timing, each of the sampling/hold circuits SA, SB, SC and SD samples and holds the detected signals, which are then respectively provided to the corresponding amplifiers GA, GB, GC and GD. According to a gain-switching signal from a terminal 14, the gains of the amplifiers GA, GB, GC and GD switch to different gains, which correspond to the foregoing detected signals when data are recorded and reproduced. Switching to different gains also causes the levels of the detected signals to have the same level when data are recorded and reproduced, which are then respectively outputted from the terminals 16A, 16B, 16C and 16D.
According to the foregoing servo signal generator, the variations in the offset voltage generated by the gain-switching of the amplifiers GA, GB, GC, GD are inputted to the servo signal calculating device. The total offset voltage of the generated servo signals is therefore affected. Particularly, in the situation of the addition signal, unlike the situation of the subtraction signal that the offset voltages can be eliminated, the influence of the offset voltages to the addition signal cannot be ignored. Therefore, the offset voltages have to be changed whenever the gain of the amplifier is changed.
Referring to FIG. 10, the detected signals A, B, C, D outputted from the amplifiers GA, GB, GC, GD are inputted to the calculating devices 18, 20. The calculating device 18 adds the detected signals A, B, C, D with a correction offset voltage Vosadd of the addition signal from the terminal 19 to generate an addition signal (A+B+C+D). The addition signal, which serves as the reflected light monitoring signal, is then outputted from the terminal 22. In addition, the calculating device 20 adds and subtracts the detected signals A, B, C, D with a correction offset voltage Vossub of the subtraction signal from the terminal 21 to generate a subtraction signal (A+B)−(C+D), which serves as the servo signal, and is then outputted from a terminal 24.
Assuming the offset voltages of the detected signals A, B, C and D respectively outputted from the sampling/hold circuits SA SB, SC, SD are Va, Vb, Vc, and Vd, and the input offset voltages of the amplifiers GA, GB, GC, GD are Vga, Vgb, Vgc, and Vgd respectively, the gains of the amplifiers GA, GB, GC, GD are G, and the calculating devices 18, 20 have no offset voltages. Under this condition, the total offset voltage of the addition signal Vadd is represented by the equation (1) below.Vadd=G(Va+Vb+Vc+Vd+Vga+Vgb+Vgc+Vgd)  (1)Namely, if the correction offset voltage Vosadd of the addition signal is −Vadd, the total offset voltage of the addition signal can be compensated.Vosadd=−G(Va+Vb+Vc+Vd+Vga+Vgb+Vgc+Vgd)However, due to the presence of the gain G of the amplifier in the above equation, there is a problem that the correction offset voltage Vosadd of the addition signal has to be changed with the gain G of the amplifier.
In addition, the total offset voltage of the subtraction signal Vsub is represented by the equation (2) below:Vsub=G[(Va+Vb+Vga+Vgb)−(Vc+Vd+Vgc+Vgd)]  (2)Namely, if the correction offset voltage Vossub of the subtraction signal is −Vsub, the total offset voltage of the subtraction signal can be compensated.Vosadd=−G[(Va+Vb+Vga+Vgb)−(Vc+Vd+Vgc+Vgd)]However, due to the presence of the gain G of the amplifier in the above equation, there is a problem that the correction offset voltage Vossub of the subtraction signal has to be changed with the gain G of the amplifier.